Field of the Invention
This invention relates generally to a corona igniter for emitting a radio frequency electric field to ionize a fuel-air mixture and provide a corona discharge, and a method of forming the igniter.
Related Art
Corona discharge ignition systems include an igniter with a central electrode charged to a high radio frequency voltage potential, creating a strong radio frequency electric field in a combustion chamber. The electric field causes a portion of a mixture of fuel and air in the combustion chamber to ionize and begin dielectric breakdown, facilitating combustion of the fuel-air mixture. The electric field is preferably controlled so that the fuel-air mixture maintains dielectric properties and corona discharge occurs, also referred to as a non-thermal plasma. The ionized portion of the fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining portion of the fuel-air mixture. Preferably, the electric field is controlled so that the fuel-air mixture does not lose all dielectric properties, which would create a thermal plasma and an electric arc between the electrode and grounded cylinder walls, piston, or other portion of the igniter. An example of a corona discharge ignition system is disclosed in U.S. Pat. No. 6,883,507 to Freen.
The corona igniter typically includes the central electrode formed of an electrically conductive material for receiving the high radio frequency voltage and emitting the radio frequency electric field to ionize the fuel-air mixture and provide the corona discharge. The electrode typically includes a high voltage corona-enhancing electrode tip emitting the electrical field. An insulator formed of an electrically insulating material is disposed around the central electrode. The igniter also includes a metal shell receiving the central electrode and the insulator. However, the igniter does not include any grounded electrode element intentionally placed in close proximity to a firing end of the central electrode. Rather, the ground is preferably provided by cylinder walls or a piston of the ignition system. An example of a corona igniter is disclosed in U.S. Patent Application Publication No. 2012/0210968 to Lykowski et al.
As shown in FIG. 1 of the '968 publication, a metal gasket provides a seal along the turnover region between the shell and insulator. However, over time, mechanical and thermal stresses wear on the gasket, such that the gasket cannot ensure a hermetic seal over the entire life of the igniter. In addition, the metal gasket does not prevent air from entering through the bottom opening of the shell and into the gap between the shell and insulator, which can lead to formation of corona discharge in the gap. A filler material, such as a resin, can be disposed between the shell and insulator to prevent corona discharge formation in the gap. However, the filler material is exposed to harsh conditions during operation of the engine and tends to degrade over time.